1. Field of the Invention
The present invention relates to the construction of a ferromagnetic laminated structure in which a laminated structure including a ferromagnetic layer and silicon is formed for injecting spin-polarized electrons from the ferromagnetic layer into the silicon, and it also relates to a manufacturing method thereof.
2. Description of the Related Art
In recent years, research and development of spin-electronic devices, which utilize both the function of spins in a ferromagnetic layer and that of electrons in electrical conduction, has been actively performed. Examples of such a device include the magnetic head in a hard disk drive and an MRAM (Magnetic Random Access Memory). Further, an idea of spin MOS-FET, which adds the function of spins to the MOS-FET (Metal-Oxide-Semiconductor-Field-Effect Transistor), has been proposed, research and development of semiconductor (silicon) spin-electronic devices being also actively performed. In this case, it is an extremely important technique to inject spin-polarized electrons into a semiconductor (silicon) from a ferromagnetic layer. In order to enhance the efficiency of injection of spin-polarized electrons into the silicon, various structures (ferromagnetic laminated structures) have been proposed.
In Non-Patent Document 1, it is theoretically predicted that, in a structure in which iron (Fe) is laminated on the (100) plane of a silicon (Si) single crystal (hereinafter, referred to as an Fe/Si (100) laminated structure), up to 100% of the spin-polarized electrons are injected into the Si. Herein, it is presupposed that the (100) plane of the Fe is grown on the (100) plane of the Si, and the [110] orientation on the Si (100) plane (hereinafter, referred to as the Si (100) [110]) and the [110] orientation on the Fe (100) crystal plane (hereinafter, referred to as the Fe (100) [110]) are arranged parallel to each other, a flat and abrupt interface being formed between these. With this combination, a large spin polarization current (a current caused to flow by the injection of spin-polarized electrons) is obtained because the symmetrical orbits of the electrons in the Fe and Si are coupled through the Schottky barrier at the interface. In other words, it has been theoretically revealed that, if the Fe (100) [110] and the Si (100) [110] are arranged crystallographycally parallel to each other, a large spin polarization current can be obtained.
However, actually, since Si and Fe have a high reactivity, it is difficult to obtain such an ideally flat and abrupt interface. Thus, actually preparing a ferromagnetic laminated structure to experimentally examine the structure is practiced.
For example, as a construction in which another substance is inserted between the Si and Fe, Non-Patent Document 2 reports the experimental result for a structure having a construction in which MgO is inserted therebetween. Herein, it has been indicated that, among the Fe layer, the MgO layer, and the Si (100) substrate, there is given a crystal orientation relationship of the Fe (100) [100], MgO (100) [110], Si (100) [110].
In this case, the plane which is arranged parallel to the Si (100) [110] is not the Fe (100) [110] disclosed in Non-Patent Document 1, but the Fe (100) [100], which is different in direction therefrom by 45°. In other words, the mechanism disclosed in Non-Patent Document 1 that gives a high spin polarization current will not be developed. In addition, in this construction, there exists a large lattice mismatch of −22.5% between the Si and the MgO, and thus at this interface, a lattice-matched crystal arrangement cannot be obtained. Therefore, this interface has a number of crystal defects which can be an electron-scatterer, thus spin-polarized electrons being scattered thereat, which makes it impossible to obtain a large spin polarization current.
Non-Patent Document 3 also discusses a FeCoB/MgO/Si laminated structure. Herein, the MgO has an amorphous structure, and it has been confirmed with a transmission electron microscope that the interface between the MgO layer and the Si is not lattice-matched. In addition, the MgO layer having an amorphous structure results in the FeCoB being polycrystalline, thus also in this case, the relationship between the Si (100) [110] direction and the Fe (100) [110] direction that is disclosed in Non-Patent Document 1 is not implemented. Furthermore, the MgO layer having an amorphous structure causes the injected electrons to be scattered in the MgO, which can have a large effect on the injection efficiency. Thus with such structure, it is impossible to obtain a large spin polarization current.
As a result of these disadvantages, no structures having the ideal interface disclosed in Non-Patent Document 1 have been actually obtained. Further, various ferromagnetic laminated structures having a structure in which an MgO layer is inserted between the ferromagnetic metal layer and the Si have been examined to be evaluated. In such a structure, crystal defects at the interface between the MgO layer and the Si or the structure of the MgO layer itself can have an adverse effect on the efficiency of injection of electrons. In other words, with conventional ferromagnetic metal/MgO/Si laminated structures, a crystal arrangement has not been obtained which is lattice-matched so as to suppress scattering of spin-polarized electrons at the laminated interface between the MgO film and the Si, thus a flat and abrupt interface structure cannot be obtained. Further, the orientation relationship between the Si and Fe crystals that has been designed from the theoretical computation in order to realize the spin injection with a high efficiency is not implemented. Consequently, such structures will not have sufficient properties for practical use as a spin injection electrode.
On the other hand, Non-Patent Document 4 discloses that, in an Fe/Al2O3/Si laminated structure, spin-polarized electrons are injected into the channel of the Si, and the resultant conduction (spin conduction) in the channel has been measured at a temperature of 10K. In this case, the Al2O3 film has an amorphous structure, and at the laminated interface with the Si, lattice matching is not obtained, thus at this interface, scattering of spin-polarized electrons cannot be avoided. Furthermore, the Fe given on the amorphous Al2O3 film is polycrystalline. Therefore, the relationship between the Si (100) [110] direction and the Fe (100) [110] direction that is disclosed in Non-Patent Document 1 is not implemented, which makes it impossible to obtain a large spin polarization current. As a result of this, the spin conduction was detected only at a temperature as low as 10K.
Further, Non-Patent Document 5 discloses that, using an Fe3Si/Si (111) laminated structure, the efficiency of injection of spin-polarized electrons was measured, the spin conduction being detected at a temperature of 50K or lower. In this case, lattice matching was obtained at the laminated interface between the Fe3Si film and the Si. However, the (111) plane of the Si is used, and the relationship that is disclosed in Non-Patent Document 1 is not implemented, the injection efficiency being lowered with the result of the detection temperature being up to 50K.
Non-Patent Document 6 discloses that, using an Fe/MgO/Si (100) laminated structure as a spin injection electrode, the efficiency of injection of spin-polarized electrons into the Si channel, and the resulting conduction (spin conduction) in the channel were actually measured. As a result thereof, the injection of spin-polarized electrons and the spin conduction were confirmed at a temperature of up to 150K. However, actually, the spin polarization rate was as extremely low as 2%, which indicates that the mechanism disclosed in Non-Patent Document 1 was not developed. Therefore, it can be estimated that the structure of the laminated film is the laminated film structure disclosed in Non-Patent Document 2 or 3.
On the other hand, Non-Patent Document 7 discloses that, using an Ni80Fe20/Al2O3/Si (100) laminated structure, the injection of spin-polarized electrons was confirmed also at room temperature (300K). However, the spin conduction was not confirmed. In other words, a ferromagnetic laminated structure having sufficient properties was not obtained. This means that, as is the case with Non-Patent Document 4, the Al2O3 film is produced as an amorphous structure, which makes it impossible to obtain lattice matching at the laminated interface of the Si, and moreover the Fe is given as a polycrystalline structure. Therefore, the requirement for orientation relationship between the Si and the Fe crystals that is disclosed in Non-Patent Document 1 is not met, which makes it impossible to obtain a large spin polarization current.